1. Field of the Invention
The invention relates to concrete finishing trowels and, more particularly, relates to a concrete finishing trowel having a drive train that is braked automatically upon the cessation of drive torque delivery therethrough. The invention additionally relates to an automatically braked gearbox usable with such a trowel and to a method of braking such a gearbox.
2. Discussion of the Related Art
Walk behind trowels are generally known for the finishing of concrete surfaces. A walk behind trowel generally includes a rotor formed from a plurality of trowel blades that rest on the ground. The rotor is driven by a motor mounted on a frame or “cage” that overlies the rotor. The trowel is controlled by an operator via a handle extending several feet from the cage. The rotating trowel blades provide a very effective machine for finishing mid-size and large concrete slabs. However, walk behind trowels have several drawbacks.
For instance, the rotating blades impose substantial torque on the cage that normally is counteracted by the operator through the handle. If the operator releases the handle while the rotor is being driven, the torque may cause the trowel to spin undesirably. In order to avoid this potential problem, some trowels employ an automatic disconnect or kill switch that shuts down the engine or otherwise stops delivery of drive  torque to the rotor if the operator releases the handle and/or if a sensor determines that the trowel is spinning at a rate indicative of loss of operator control. A trowel employing an automatic engine shut-down mechanism is disclosed, for example, in U.S. Pat. No. 2,734,932 to Barnes. A trowel that relies on release of a dead man lever or similar structure to shut down a trowel's engine or disengage a clutch upon operator release of a steering handle is disclosed in U.S. Pat. No. 4,629,359 to Sengupta.
However, merely shutting down the engine or otherwise ceasing the delivery of drive torque to the trowel's rotor does not necessarily prevent the trowel from spinning because the handle and cage have considerable momentum at the time of shut down. The trowel may spin through a substantial arcuate range of a complete revolution or more while that momentum is being spent. This slow response to an attempted shut down is undesirable.
Attempts have been made to mitigate this problem through the incorporation of active brakes in the trowel's drive train that are designed to prevent or at least inhibit rotation of the trowel's frame.
For instance, U.S. Pat. No. 4,280,980, assigned to Stone, discloses a combined clutch/brake coupling a drive belt of the trowel's drive system to the input shaft of the trowel's gearbox. The clutch must be manually-engaged by displacing a lever mounted on the handle. The clutch may be manually disengaged by manual operation of the handle. It may also be disengaged automatically through the operation of a centrifugal weight that is responsive to undesirable rapid spinning of the trowel. In either event, movement of the handle to the clutch disengaged position activates a spring-applied pad-type brake that acts on the drive belt. Although this device works reasonably well, it  exhibits several drawbacks. For instance, it is usable only with a manually actuated clutch that is actuated by the operator only at the expense of release or partial release of the handle. The clutch activation lever also is open to the outside, exposing the internal components of the clutch to contamination by dirt, debris, water, etc. In addition, the clutch brake engages automatically only if a mechanical sensor indicates that the trowel is undergoing unacceptable centrifugal forces. It does not react to an “ordinary” engine shut down situation in which torque transfer to the rotor ceases in the absence of such centrifugal forces. The brake also imposes significant drag on the drive belt, accelerating wear on the drive belt. Finally, braking forces imposed on the clutch are generated solely by the strength of the spring and, therefore, are independent of backdrive forces imposed on the system's drive train by the rotor.
Other brakes are available for other applications that brake a clutch directly rather than braking a drive belt leading from the clutch to the gearbox. Known clutch brakes lack some of the drawbacks of the Stone clutch brake but have drawbacks of their own, rendering them poorly suited for use with a trowel. One such clutch brake is manufactured by North American Clutch Manufacturing (NORAM). The NORAM brake is a manually engaged brake taking the form of a deadman's switch operated by a control lever much like that commonly found on a walk behind lawnmower. The brake is engaged automatically when the operator releases the control lever to brake the clutch. This brake also works reasonably well, but requires that the operator squeeze the control lever at all times while operating the trowel. This continuous squeezing requirement leads to considerable operator fatigue and also requires that the operator divert a substantial portion of his or her attention to operation of the control lever, hindering his  or her ability to adequately steer the trowel. In addition, the NORAM clutch brake, like that employed by the brake of the Stone machine, imposes braking forces that are independent of the magnitude of backdrive forces generated by the machine's output. In addition, as with the Stone system, the range of clutches useful with this type of system is considerably limiting.
Still another type of clutch brake, offered by Ogura, is electrically powered. An electrically powered clutch brake requires the presence of a reliable power supply to prevent the brake from wearing or seizing. The magneto employed as an electrical power source for a typical walk behind trowel is ineffective for the purpose. Adding additional components to the trowel to upgrade the power supply would add additional cost and weight to the trowel.
The need therefore has arisen to automatically brake a drive train of a concrete finishing trowel upon the cessation of drive torque delivery to the trowel's rotor without interfering with the operator's ability to steer and control the trowel and without significantly adding to the cost or complexity of the trowel.
The need has also arisen to provide a drive train brake that satisfies the precedingly described need and that does not significantly add to the cost or weight of the trowel.
The need has additionally arisen to provide a brake that satisfies the first-mentioned need while still being compatible with a variety of different drive systems. 